The storm over Kevin Brooks's Carnegie medal win has gathered force after two more writers criticised the choice, saying it called into question the whole purpose and value of the UK's top children's book prize.

The award-winning children's author and scriptwriter Frank Cottrell Boyce and the critic Amanda Craig are the latest to attack the award – which has been made annually by librarians since 1936 – joining Telegraph reviewer Lorna Bradbury, who described the winning book as "a uniquely sickening read" which "seems to have won on shock value rather than merit".

Writing in the Independent, Amanda Craig said she had refused to review The Bunker Diary on its publication, because it was "depressing both in its nature and its lack of redemption".

She added: "It is deeply ironic the Carnegie should have chosen this one, out of an otherwise engaging oeuvre, to celebrate and promote. It is the latest in a trajectory for the Carnegie prize which nobody who loves children's books can possibly applaud."

Cottrell Boyce chose to enter the fray via a comment on the Guardian news story. He wrote: "I'm not making a comment on Brooks's book which I haven't read. He is well-regarded and the Carnegie should reward excellence."

He continued: "There is however a problem here. The Carnegie was instituted as a prize for children's fiction. Brooks is a YA writer. YA fiction is extremely lucrative for publishers. It sells well and is low-risk as the vast majority of titles - including some brilliant books - are generic (vampires, sick kids, issues etc).

"Could not some of the publishers who have done so well out of the category stump up for a YA prize instead of predating on one of the few places where children's books aimed at children can still get some attention?"

The Bunker Diary is the story of six people, including the 16-year-old diarist Linus and a nine-year-old girl, who have been kidnapped and locked in a bunker. Reviewing it for the Guardian, Simon Mason described it as "a horrific parody of a reality TV show".

Opinions among the book's target readers appeared to be mixed, with one parent commenting on the Guardian's news story that The Bunker Diary had "freaked out" her 12-year-old daughter and sickened and disturbed her 15-year-old son. "This book should have a warning on the cover, and should not be allowed to be read by children under 16 years of age."

However, the winning book got strong support on the Guardian's children's books site from school reading group, The Millennium RIOT Readers, from Charles Thorp Comprehensive School in Gateshead. "The Bunker Diary is very bleak and unusual, but the characters in the story are what made this book important," they wrote. "Their breakdown, because of the helplessness of the situation they find themselves thrust into, is what, we felt, drives the story forward to its frustrating and thought-provoking end."